Conferencing systems have become popular in the telecommunications art over the past several years. Many such conferencing systems are used to implement conferences that involve entertainment types of services whereby a relatively large number of conferees may speak to one another for a fee which is set and advertised by the service provider. Common examples are dating services which are often advertised on television, whereby numerous conference conferees can telephone in and speak to one another. These conferences are implemented using a device known as a conference bridge, the purpose of which is to interconnect a plurality of conferees so that the audio signal transmitted to each conferee is effectively equal to the sum of all audio signals transmitted from the other conferees.
Other applications envisioned include remote stockholder meetings, distance learning, technical training, and any other scenario in which a plurality of conferees are interconnected.
During such conferences, the relatively large number of conferees varies as conferees enter and exit the conference call. The conference bridge includes appropriate control functions to allow conferees to enter and exit from the conference. These control functions of the conferencing bridge are typically invoked by the conferees entering Dual Tone Multi-Frequency (DTMF) tones in order to enter or exit various conferences, and to switch among the conferences. Unfortunately however, the entry of these tones is conveyed to the other conferees since a tone signal appears to the conference bridge just as any other audio signal. When the number of conferees is relatively large, this phenomena results in annoying tones being conveyed to the conferees on a relatively frequent basis. For example, on a commercial party line interconnecting thirty-two conferees, someone may enter or exit the conference every few minutes.
Additionally, DTMF tones can be used to control volume or any other function of the system. This fact further increases the frequency with which DTMF tones are generated by conferees.
While there exist some prior attempts at solving the above problem, these solutions give rise to other problems of their own. For example, U.S. Pat. No. 5,327,492 issued to Parola describes a system whereby a buffer is utilized to detect and block DTMF tones. However, in order for any detection algorithm to detect such tones, the buffer must have sufficient length, and therefore introduces a noticeable delay in the signal path. While the delay in and of itself is undesirable, the more noticeable problem is that the echoes normally present in such a system sound much worse to the conferees if the echo signal passes through a relatively long delay than they do if the delay is short. Thus, when a system designer seeks to minimize the effects of echo, delay should be minimized.
As is known to those skilled in the signal processing and telephony art, and as can be appreciated from the above, there are two competing interests in implementing prior art devices such as the Parola technique. First, in order to accurately detect the presence of DTMF tones, it is required that there be some signal history for processing. Hence, a buffer is introduced and the reliability of the tone detection increases with the length of the buffer. It would appear therefore, that a longer buffer is desirable. However, a countervailing interest is the minimization of the effect of echoes and the delay experienced by the signal. As the buffer length is increased, the delay and effect of the echoes increases, which results in degraded performance noticeable by all conferees.
In view of the above competing interests, a typical approach in the art is to try to trade off the two requirements so that the buffer length is both long enough to provide acceptable tone detection and tone blocking performance, while at the same time being short enough such that significant delay and the resulting negative impact upon the echo signal are avoided. It can be appreciated however, that perfect performance with respect to either of these competing requirements cannot be achieved.